This invention relates generally to apparatus and methods for obtaining information used to measure electrical properties of a formation intersected by a well borehole. More particularly, but not by way of limitation, the present invention relates to apparatus and methods for concurrently detecting and measuring a plurality of currents which are focused as they flow from a plurality of electrodes, mounted on a single equipotential housing, in response to a single voltage signal from a single power supply so that electrical characteristics of the formation, such as resistivity, can be concurrently determined at different depths of investigation.
It is known that measurements of formation resistivity made at several lateral distances from a well borehole provide information which can be used in analyzing whether the formation has suitable characteristics for likely producing oil and gas. A deep measurement will give apparent formation resistivity, a shallow measurement will give the invaded zone resistivity, and a very shallow measurement of the formation immediately adjacent the borehole will give flushed zone resistivity.
One technique for measuring the resistivity of a formation is with focused currents. This technique forces a current flow in a relatively limited width laterally or radially outwardly from a suitable focused-current apparatus disposed in a borehole. One specific type of such an apparatus is described in U.S. Pat. No. 4,122,387 in the name of Ajam et al. This apparatus includes a tool having a shallow guard tool connected with, but electrically insulated from, a deep guard tool. This apparatus has two power supplies and two current return electrodes for simultaneously logging two focused currents which are making two different depths of investigation in the surrounding formation. A depth of investigation refers to the lateral outward extent from the borehole into the formation within which a certain percentage of the tool response is developed, as known to the art.
The type of tool disclosed in the Ajam et al. patent has several shortcomings. For example, the tool of the Ajam et al. patent is basically two separate tools mechanically connected, but electrically insulated from, each other. Additionally, the Ajam et al. apparatus has a power system for each guard section of the tool. There is also one current return for each guard section. Such duplicated construction makes it difficult to add more sections for simultaneously logging more than two depths of investigation.
There is the need for an apparatus and method by which multiple depths of investigation can be simultaneously logged without requiring substantial duplication of parts for each additional depth of investigation. It is also desirable for such an apparatus to have a simplified construction wherein a single fully integrated tool, rather than two separate tools simply connected together, is obtained. However, such an integrated tool must be operable and provide suitable focused currents.